1. Technical Field
The present invention relates to a technique for measuring an impedance by supplying a test signal to an external load. Also, the present invention relates to a technique for adjusting an amplitude of an output signal in accordance with the external load.
2. Background Art
In a sound output circuit that amplifies a sound signal and outputs the amplified signal to an external load, one which measures an impedance of the external load and switches between sound signals in accordance with a measured value of the external load, is known (JP-A-2003-179441). A speaker or a headphone to be connected as the external load, can have an impedance of 4, 8, 16 or 32Ω. In a conventional technique, since a sound signal is switched in accordance with the external load connected to the sound output circuit, the technique has an advantage that it is possible to prevent a device connected to the sound output circuit as the external load from being damaged by an excessive input, in advance.
Meanwhile, it is thought that a test signal is supplied to an external load in order to measure an impedance of the external load. In this case, an output signal and the test signal are switched to be supplied to the external load.
However, from a view point of protection of the external load, it is general that the output signal is supplied to the external load through an internal load such as a protection resistor or the like in a signal amplifying device. In this case, when the test signal is supplied to the external load through the internal load, a problem arises that a total impedance of the internal load and the external load is to be measured so that the impedance of the external load is not accurately measured.
Also, in a signal amplifying circuit that amplifies an input signal and outputs the amplified signal to an external load, a technique is known in which a test signal is supplied to an external load and determination is made as to whether the external load is a stereo plug or a monaural plug (JP-B-4182802). In the technique, a frequency of the test signal is set to a high frequency out of an audible band so that a user does not hear a sound of the test signal.
A speaker or a headphone to be connected as an external load has a reactance component so that an impedance thereof varies due to a change in a frequency. Therefore, when a test signal is set to a high frequency out of an audible band, the impedance in the high frequency may be possibly deviated from an impedance in the audible band. Consequently, the impedance in a required audible band is not able to be accurately obtained. In addition, the test signal may be attenuated in the external load so that it may become difficult to measure the impedance at a high SN ratio. Moreover, it is necessary to expand a frequency characteristic of a circuit for generating the test signal up to a high frequency out of an audible band so that it is necessary to cause components constituting a signal amplifying circuit such as transistors to correspond to the high frequency.